Acute decrease by human chorionic gonadotropin of the activity of preovulatory ovarian 17 alpha-hydroxylase and...

Female rats were treated with 10 IU pregnant mare's serum gonadotropin (PMSG) on the 21st day after birth, and 53 h later with 20 IU human CG (hCG) in order to investigate the mechanism of action of hCG upon ovarian 17 alpha-hydroxylase and C-17-C-20 lyase. The activity of ovarian 17 alpha-hydroxylase and C-17-C-20 lyase was severely decreased within 6 h after the injection of hCG. However, treatment with cycloheximide or puromycin before the hCG injection prevented the decrease in the activity of these enzymes by hCG. Actinomycin D pretreatment prevented the hCG-induced decrease of C-17-C-20 lyase activity. In correlation with the changes in these enzyme activities induced by hCG, the concentration of ovarian microsomal cytochrome P-450 decreased to barely detectable level after hCG treatment; this decrease was also prevented by pretreatment with cycloheximide. When the cytochrome P-450 obtained from the PMSG-treated rats was subjected to spectrometric analysis of binding with progesterone and 17 alpha-hydroxy-progesterone, type I spectra were obtained for both steroids, indicating that the steroids bound to the cytochrome as substrate. Testosterone, a substrate for aromatase, showed very limited binding to the cytochrome, however. On the other hand, the microsomes obtained from the PMSG + hCG treated ovaries showed no type I spectrum with progesterone and 17 alpha-hydroxy-progesterone. The activity of NADPH-cytochrome c reductase in the ovarian microsomes was not significantly influenced by the hCG treatment. These results indicate that the acute decrease of ovarian 17 alpha-hydroxylase and C-17-C-20 lyase activity by hCG treatment was caused through the decrease of cytochrome P-450 and is correlated with de novo synthesis of RNA and protein.